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X-ray CT images and implications

Half-round cores from Hole U1316A with a total length ~6 m (Sections 307-U1316A-6H-4, 0 cm, through 7H-1, 54.5 cm) were subjected to X-ray CT analysis. To save measurement time, X-ray CT measurements were performed with several cores mounted on the stage at the same time divided by boards and styrene foam (Fig. F1). Total data acquisition time with sample preparation was ~10 h for these samples. After the CT data were taken, cores were sent back to the Bremen Core Repository for further studies in the future. The obtained raw X-ray CT slice image contains several pieces of information (Fig. F2). Core data were extracted from this raw data. An example of a set of slices of X-ray CT images is shown in Figure F3.

The intensity of the transmitted X-ray beam is usually expressed as CT number (Hounsfield, 1973), which is the ratio of the linear attenuation coefficient of the material to that of pure water. Histograms clearly show significant difference in CT numbers among air, board, liner, water, sediments, and pebbles (Fig. F4A). Figure F4B is a close-up of a histogram to enhance distribution of denser materials.

Images on the yz and xz planes, as defined in Figure F5, are shown in Figure F6, although any angle could be displayed. The stage stroke of the X-ray CT system prohibits measuring 1.5 m long whole sections at the same time, which results in the same section measurements being divided by two. A distortion with increasing distance from the horizontal plane during X-ray CT measurements occurs, which is most readily observed as a misfit between adjacent sets of slices. Therefore, Sections 307-U1316A-4H-4, 4H-5, and 4H-6 are shown in two parts, which overlap by a few centimeters.

These images are mainly compared with digital images of the split surface, which shows good correlation on the basis of cracks, lithological boundaries, and sedimentary structures. Sedimentary structures, such as various types of lamination and shape, occurrence, and textural relation to surrounding host sediments of dropstones and pebbles, are easily recognized on X-ray CT images. Since X-ray CT numbers eventually depend on the density of a substance and densities of coral-bearing and surrounding sediments are nearly the same, coral fragments would not be well distinguished from surrounding sediments on X-ray CT images. Carbonate cementation may render density distributions that reflect sedimentary structures as homogeneous, preventing clear imaging of these sedimentary structures. It may also relate to resolution of the X-ray image. On the other hand, X-ray images can reveal sedimentary and postsedimentation structures and materials that cannot be seen on the split core surface.

Cracks have widened since the core photos were taken. It is notable that some cracks seen in the X-ray CT images were not clear on core photos. This means that cracks formed and grew with time, especially in coral-bearing layers. Drilling disturbances close to the liners were frequently observed.

Section 307-U1316A-6H-4

This section is shown in Figure F6A, F6B, and F6C. The upper 20 cm of this section consists of brown silty clay, which can be recognized on X-ray CT images. This section also contains the boundary between lithologic Subunits 2A and 2B as shown by the yellow dashed line in Figure F6A and F6B. Subunit 2A is dominated by siliciclastic fine to medium sands, whereas corals predominate in Subunit 2B. However, densities of coral-bearing layers and surrounding sediments are similar, which prevents clear discrimination on X-ray CT images. X-ray CT images clearly map dropstones, and some appear in the core photo within Subunit 2A. In the lower part of this section, a dense rudstone rich in highly fragmented corals is observed.

Section 307-U1316A-6H-5

This section is shown in Figure F7A, F7B, and F7C. In the upper part of this section, from 0 to 13 cm, a dense rudstone rich in highly fragmented corals is recognized on X-ray CT images. Corals occur in distinct layers interbedded with siliciclastic sediments, also seen clearly in X-ray CT images.

Section 307-U1316A-6H-6

This section is shown in Figure F8A, F8B, and F8C. Several dropstones/​pebbles (at 67 cm in Fig. F8A, F8B) clearly captured in X-ray CT images cannot be seen on the split core surface. Some sedimentary structures and drilling disturbances are evident in X-ray CT images.

Section 307-U1316A-6H-7

This section consists of L. pertusa rudstone with silty clay matrix (see the “Expedition 307 summary” chapter). The X-ray CT image clearly captures one pebble/​dropstone (at 52 cm in Fig. F9) and its relationship to the surrounding host sediments.

Section 307-U1316A-6H-CC

This section (Fig. F10) consists of L. pertusa rudstone with silty clay matrix (see the “Expedition 307 summary” chapter). X-ray CT images suggest that cracks divided this section into four segments and lower density material appears below these cracks.

Section 307-U1316A-7H-1

At the top of this section (Fig. F11), the core photo shows pebbles with diameters from 0.5 to 2 cm, which are also observed in CT images. A histogram of CT numbers shows that this section contains denser materials (Fig. F4A, F4B). This information from X-ray CT images is consistent with the core photo. The CT image shows a major unconformity located between lithologic Units 2 and 3. Also, the CT image clearly distinguishes the uppermost 10 cm of Subunit 3A, which shows gradual change from light gray to greenish gray. Below this level, the subunit is characterized by generally homogeneous sediments (see the “Expedition 307 summary” chapter).